Heat-recoverable articles, such as heat-shrinkable tubes and heat-shrinkable caps, are used as covering for joints between insulated wires, wire terminals, and metal pipes for the purpose of waterproofing, corrosion protection, and the like. For example, when a joint between insulated wires is covered with a heat-shrinkable tube and heating is performed, the heat-shrinkable tube shrinks along the shape of the joint and adheres thereto, and thus the joint can be protected.
Examples of such heat-recoverable articles include a multilayered heat-recoverable article in which an adhesive layer is provided on the inner surface of a heat-shrinkable base material layer (refer to Japanese Unexamined Patent Application Publication No. 2000-129042). As the adhesive layer, a hot-melt adhesive, such as an ethylene-vinyl acetate copolymer (EVA), an ethylene-ethyl acrylate copolymer (EEA), or a polyamide resin, is used. Furthermore, the adhesive layer may be formed so as to include two thermoplastic resin layers (refer to Japanese Unexamined Patent Application Publication No. 8-230037 and Japanese Patent No. 4019524).
On the other hand, a multilayered heat-recoverable article is produced by a method in which a heat-shrinkable base material layer and an adhesive layer are each extruded into a tubular shape, then the resulting tubular body is expanded in the radial direction (the diameter is expanded) under heating, and cooling is performed to fix the shape. In some cases, in order to improve heat resistance, the extruded multilayered heat-recoverable article may be irradiated with ionizing radiation so that the heat-shrinkable base material layer can be crosslinked.
When the multilayered heat-recoverable article is used, by performing heating with an adherend being covered with the multilayered heat-recoverable article, heat shrinkage of the heat-shrinkable base material layer occurs, and at the same time, the adhesive layer is fluidized. At this time, the space between the adherend and the heat-shrinkable base material layer is filled with the fluidized adhesive layer, and thereby, the multilayered heat-recoverable article is made to adhere closely to the adherend.
In the case where the multilayered heat-recoverable article is made to adhere to an insulated wire, which is an example of the adherend, in which a conductor is covered with an insulating resin layer, the adhesive layer of the multilayered heat-recoverable article comes into contact with the insulating resin layer. When the insulating resin layer and the adhesive layer are in contact with each other, depending on the types of resin and additive used in the insulating resin layer and the adhesive layer, there is a possibility that the insulating resin layer of the insulated wire will be deteriorated. Consequently, in order to avoid deterioration of the insulating resin layer, it is necessary to appropriately select the combination of the insulating resin layer and the adhesive layer, and the type of adherend for which the multilayered heat-recoverable article can be used becomes restricted. As a result, the user has to select a multilayered heat-recoverable article that is unlikely to be deteriorated according to the type of adherend, such as an insulated wire, which is inconvenient.